This invention relates to telecines, namely apparatus for generating video signals from cinematographic film.
The telecine field is fully described in TV and G. B. Video Engineers Reference Book, ed. K. G. Jackson and Townsend, ISBN 0-7506-1021-2, see Chapter 39 by J. D. Millward, to which reference should be made for a background description of this art.
Telecines are broadly of three types, namely camera-tube, flying-spot, or ccd (charge-coupled device) or solid-state, depending on the scanning system used. They Can also be continuous motion, in which the film moves at a steady rate, or intermittent motion in which the film moves in a gate as in an optical projector.
The present invention will be described for simplicity in the context of a continuous-motion flying-spot telecine, this being the area in which it is most important, though it could be applicable to other types of telecine.
In order to provide video signals for use in high-definition television (HDTV), higher scanning rates are required than with conventional 625/50 or 525/60 standards. To produce an HDTV signal with interlaced scanning, the requirement is for a flying spot telecine with a bandwidth of 30 MHz, whereas the current high definition flying spot telecine technology is only capable of 20 MHz.
This limitation arises due to the afterglow of the fluorescent phosphor used in the cathode ray tube (CRT) of the flying-spot telecine. At the lower frequency of 20 MHz the effective amplitude response of the phosphor is 12%, where 100% is theoretically perfect response. The signal amplitude has to be boosted to 80%, and the signal-to-noise ratio is then just about acceptable. However at a frequency of 30 MHz, the phosphor response is only 3%. A larger still amount of amplification would be required, which would also amplify the noise, and the signal-to-noise ratio would become unacceptable.
Attempts to find a fluorescent material with a shorter afterglow characteristic, to give an improved response at higher frequencies, have not so far met with significant success.
It is well-known in telecines to provide for scanning of the film more than once. In particular, this is required for direct production of an interlaced scan signal. It is in any event required to produce sufficient film frames, given that the conventional film frame rate is 24 frames per second whereas the conventional television field rates are 50 or 60 fields per second. Conversion to 50 fields per second is achieved by running the film at 25 frames per second, and scanning each frame twice, whereas conversion to 60 fields per second is achieved by scanning one frame twice and the next three times, and so on.
The twin-lens continuous-motion flying-spot telecine in particular was designed for the situation where two television fields are required from each film frame. Two objective lenses are provided, spaced in the direction of movement of the film. The first television field is produced using the first lens and the second television field is produced using the second lens. The lenses are thus used alternately. A single set of photosensors is used to provide a video output for both scans.
Split optical systems, of which the twin-lens telecine is one example, have been known for very many years, see for instance British Patent Specifications GB-A-475 032 (1937) and 494 365 (1938). However, in such systems the same film frame is scanned twice and the scan that is to be operative at an instant is selected, the other being inhibited or discarded.
British Patent specification GB 1 232 133 discloses the use of a twin-lens telecine having two photo-sensitive detectors to detect light passing through two adjacent film frames. The outputs of the detectors are read alternately so that the film can be run with an intermittent pull-down; the output of one detector being read during pull-down intervals and that of the other detector inbetween the pull-down intervals.
It is also known, from for example, British Patent Specifications GB-A-1 535 563 and 1 542 213, to provide a frame store at the output of a telecine. This enables the film to be scanned with a "progressive" scan, that is a sequential or non-interlaced scan, and then converted electronically to interlaced form by means of the frame store.